As systems, such as multimedia entertainment, communications, process control systems and diagnostic systems utilized by the automotive and aerospace industries, become more complex, a need arises for additional devices to communicate, either with each other or with a central controller or the like. Historically, these systems included dedicated wiring extending between the various devices in order to support communications therebetween. As systems have become more integrated and the communications requirements have been increased, the amount of dedicated wiring that would be required can quickly become excessively large, both in terms of the space required for the wiring and the cost of the wiring and the attendant installation. Moreover, as the amount of dedicated wiring increased, the overall complexity of the system also generally increased as well as the likelihood that some portion of the wiring might be damaged or broken during or following installation.
As such, network busses have been developed to provide a common communication path between a plurality of devices. In automotive and aerospace applications, for example, a network bus can be utilized to monitor various components and to collect diagnostic and status information. In this regard, diagnostic and status information relating to the strain, acceleration, pressure and/or temperature to which the various components are subjected may be collected and analyzed. By way of further example, a network bus architecture is currently being developed to support process control applications, as wells communications and the delivery of multimedia information to the occupants of a vehicle, such as an automobile, minivan, sports utility vehicle, aircraft, boat or the like. Advantageously, this network bus would transport the audio signals, including streaming audio signals, produced by one or more of a radio, a cassette tape player, a compact disc player or the like to selected speakers or headphone jacks throughout the vehicle. Similarly, the network bus may support voice and data communications with a cellular telephone carried by an occupant of the vehicle, as well as communications with a laptop computer, a handheld computing device or the like. Also, the network bus may transmit video signals, including streaming video signals, from a television receiver, a videocassette recorder or other video source to one or more video monitors. In addition, the network bus may transmit sensor and actuator signals to and from devices such as drivetrain devices, passive restraint devices, crash avoidance devices, drive-by-wire devices, or the like.
In addition to the variety of devices that are connected to a network bus, one or more controllers are also generally connected to the network bus for receiving data from the various devices and for sending commands to the devices. Among other things, these commands specify the manner in which the various devices are to function including the manner in which the various devices are to transmit information over the network bus. Additionally, the controller(s) can receive input from an operator, such as an occupant of the vehicle. This input can include, for example, an indication of the source(s) of the signals to be transmitted over the network bus as well as the destination of the signals.
Traditionally, networks of the type described above have transmitted data in analog format. Unfortunately, analog signals are susceptible to noise introduced into the signals during data transmission. Given that many of the transmitted signals have a low amplitude to start with, this noise can corrupt the signal and decrease the signal to noise ratio to levels that cause loss of resolution in the signal. Further, as many of these network devices are scattered some distance from the controller, the electrical lines connecting the network devices to the controller may be sufficiently long to cause signal degradation due to DC resistance in the wiring.
In light of these shortcomings, it would be advantageous to utilize digital networks. But, many conventional digital networks suffer from a variety of problems themselves. For example, many existing digital networks operate according to complicated protocols which require each network device to have a relatively high level processor, thereby increasing the cost of the network devices. Complicated protocols also introduce overhead into the messages on the bus that are not necessary for data acquisition and control. This overhead can severely limit the number of data samples that can be transmitted on the bus. These networks also have other problems. For example, they generally do not support both acquisition and control, and they typically only support networks that extend over relatively short lengths. Further, these networks typically have bulky network device interfaces, slow network data rates and/or a low network device count. Additionally, many computer systems that include digital networks do not operate in a time-deterministic manner. As such, these computer systems generally lack the capability to schedule a trigger command to the network components that repeats or is interpreted and executed with any precision timing.
Regardless of the digital or analog nature of network, the network bus may be damaged during or following installation. In this regard, the network bus typically consists of a plurality of conductors or wires that may extend great lengths between the various controllers and network devices. Due to accidents or other unforeseen circumstances, one or more of the wires may be broken, thereby creating an open circuit. Thus, components on one side of the open circuit will be unable to communicate via the broken conductor with components on the other side of the open circuit. Additionally, signals transmitted over the broken conductor will be reflected by the broken end of the conductor due to the characteristic impedance mismatch. The reflected signals will then be returned along the conductor, thereby interfering, both constructively and destructively, with other signals being transmitted via the conductor. While the components on one side of the open circuit may be able to communicate at relatively low data rates, such as ten kilobits per second, reflected signals will generally prevent effective communications between the components at higher data rates such as ten megabits per second or the like. The destructive interference caused by the reflected signals caused by the open circuit will render the signals on the broken conductor more susceptible to noise, thereby further limiting effective communications.
In instances in which one or more conductors of the network bus are broken, one of two different approaches has generally been taken. According to one approach, the network bus remains unrepaired for at least some period of time and communications continue over the network bus, albeit at a relatively slow data transfer rate that is selected so as not to be corrupted by the reflected signals. Since a number of applications require that communications be conducted via the network bus at relatively high data transfer rates, the intentional slowing of the data over the network bus to reduce, if not negate, the deleterious impact of reflected signals may be inappropriate. Alternatively, communications via the network bus can be halted and a technician or repair person can troubleshoot the network bus to identify the break in the network bus and can then physically repair the broken conductors. Once the repairs have been completed, the communications over the network bus can be recommenced. However, the physical repair of the network bus oftentimes requires that the network bus be removed from service for some period of time, which action may also be inappropriate for certain applications, such as time-sensitive applications or other applications that demand continuous monitoring or feedback.
A network bus may have other types of failures in addition to failures attributable to an open circuit. For example, one or more of the pairs of conductors that comprise the network bus may develop a short circuit. In this instance, the network controller and the network devices will no longer be able to communicate via the pairs of conductors that are shorted. As such, the network bus would have to be removed from service, the location of the short circuit would have to be identified, and the network bus could be repaired, prior to recommencing communications via the network bus.
In addition to short circuit and open circuit conditions on the network bus, a network device that is electrically connected to the network bus may fail and, as a result, may create problems for the network bus and for other network devices connected to the network bus. In this regard, some failure modes of the network devices are self-limiting in that failure of a network device does not adversely impact continued communications of the other network devices over the network bus. However, other failure modes of the network devices may create problems on the network bus and effectively prevent other network devices from communicating via the network bus. One example of a failure mode of a network device that creates problems on the network bus is exhibited in instances in which a network device that fails emits a stream of meaningless data onto the network bus. In this situation, the network device is typically described to be “babbling.”
A network device that is babbling can monopolize control of the network bus and can prevent the network controller and the other network devices from communicating via the network bus. In this situation, the network device that is babbling must be identified and removed from the network bus in order to permit the other network devices and the network controller to communicate via the network bus. As will be apparent, the process of identifying the network device that is babbling and thereafter removing the babbling network device from the network bus can be time-consuming, during which time the network bus will be unavailable for communications between the other network devices and the network controller.
Yet another type of failure of a network device that could cause the whole network to fail is that of a short circuit condition inside the network device that causes it to draw too much current. If the network device draws more than the amount of current that can properly be supplied by the power supply in the network controller and power conductors of the network bus, the power voltage supplied to the other network devices will drop to levels that will cause them to be inoperable. As will be apparent, the process of identifying the network device that is shorted and thereafter removing the shorted network device from the network bus can be time-consuming, during which time the network bus will be unavailable for communications between the other network devices and the network controller.
As described above in conjunction with the repair of a network bus having an open circuit condition, the removal of a network bus from service in order to repair a short circuit condition or to remove a network device that is babbling from the network bus will require that the network bus be out of service for some period of time and would therefore be inappropriate for certain applications including time-sensitive applications or other applications that demand continuous monitoring or feedback.
Accordingly, it would be advantageous to develop an improved network bus that could accommodate bus failures caused by open and short circuit conditions and by network devices that begin babbling. Moreover, it would be desirable for the improved network bus to support continued communications between the devices connected to the network bus without having to slow the data transfer rate and without having to remove the network bus from service in order to physically repair the network bus.